First of all you must say what's the distance from traffic sign to the axis of the train (ag).
Then you have to consider the aeredynamics of your train (K = aerodynamic factor).
For examples:
ag = 2.3 meter
q1k = 1.75 kN/m2 (wind load)
k = 1 (bad aerodynamic)
= 0.85 (quite good aerodynamic)
= 0.60 (good aerodynamic)
q = k*q1k (wind load)
bye bye
Ing. Nicola